Optic Nerve Sheath Meningioma

What is it?

Benign tumor growing from the meninges of the orbital portion of the optic nerve
Compresses the intraorbital and/or intracanalicular optic nerve to cause painless progressive monocular visual loss
Affects mostly young and middle-aged women
Biopsy may worsen vision permanently
Radiation therapy may prevent visual decline

Core clinical features
- Slowly progressive painless monocular visual loss
- Minimal if any proptosis
- Afferent pupil defect on the affected side
- Nerve fiber bundle defect and sometimes reduced visual acuity
- Optic disc appears normal, elevated (sometimes with shunt vessels), or pale
Possible accompanying clinical features
- Increased resistance to retropulsion of the affected eye

Imaging features
- MRI shows a linear or lobular mass around the orbital optic nerve
Trap: this tumor may go undetected on imaging protocols that do not include contrast and do not adequately visualize the orbit
Trap: distinguishing chronic dural inflammation and orbital lymphoma from optic nerve sheath meningioma is difficult by MRI alone

Order orbit-based MRI
Base your diagnosis presumptively on imaging, as biopsy is too destructive of vision
Tip: CT is useful to show calcium in the lesion, which would be more typical of meningioma than of dural inflammation or lymphoma
Tip: an empirical trial of prednisone 1mg/kg/day for 14 days is also useful because meningioma is hard to distinguish from dural inflammation or lymphoma; visual improvement suggests dural inflammation or lymphoma rather than meningioma
If vision does not improve with prednisone treatment, recommend radiation therapy if
- Vision is still reasonably intact in the affected eye
- Vision is worsening in the affected eye
- Unaffected eye has poor vision

Visual loss spontaneously worsens in 75% and remains stable in 25%
Tip: further intracranial tumor growth that compromises the fellow optic nerve or the optic chiasm is low and should not be used to justify radiation therapy
Radiation therapy improves vision in 10%, stabilizes vision in 80%, but does not prevent continued vision loss in 10%
Late visual loss or other complications of radiation therapy are rare if proper technique is applied
Trap: inexpert radiation therapy may cause a severely dry eye and loss of vision from infarction of the optic nerve and bleeding in the retina, often months to years later